1. The Field of the Invention
The invention relates generally to methods for producing and utilizing heat by oxidizing fuels without mixing the fuels with air and thereby producing a fire. More particularly, the invention is directed to an autothermal process for fuels conversion, including methods and systems for generating heat by oxidizing fuel without mixing the fuel with air, with the subsequent transfer and utilization of the generated heat being an improvement over the heat transfer which is possible when fire is used to produce heat.
2. The Relevant Technology
Historically, the primary method by which mankind has used fuel to generate heat has been fire. For many applications, however, the use of fire to produce heat has a number of substantial disadvantages and limitations. One of the limitations of fire is that mixtures of fuel and air must contain more than some critical amount of fuel in order to burn. This is the well known flammability limit.
Another of the limitations of fire relates to its thermodynamics, i.e., fire is an irreversible process. While energy can neither be created nor destroyed, it can become less available for doing useful work. A fuel contains chemical energy, some fraction of which is potentially available to do useful work. Upon combustion of a fuel by fire, that chemical energy is converted into heat energy. The fraction of this heat energy which is potentially available to do useful work is less than the fraction of the chemical energy which was potentially available to do useful work.
A further disadvantage of fire relates to heat transfer. For all fuels in common usage, combustion produces hot gases. In many applications it is necessary to recover heat from these hot gases. This is commonly done by passing the hot gases over heat transfer surfaces, but the amount of heat that can be transferred between hot gases and a fixed amount of solid surface is generally relatively low. Thus, to recover the heat efficiently, large amounts of heat transfer surface are needed. For industrial processes using fire as a heat source, the cost of providing heat transfer surfaces to recover the heat is frequently a major part of the total process cost.
Fire also has the property of being an intense phenomenon. For a flame to sustain itself, large amounts of heat must be liberated at very high temperatures with a very high rate of heat release. For many applications heating in a more controlled manner is needed. For these applications electrical heating is frequently used.
In efforts to overcome one or another of the disadvantages of fire, a number of alternatives to fire have been proposed. The flammability limits are a problem in some situations, i.e., there are industrial operations which produce mixtures of one or more toxic organic materials with air. These mixtures must be disposed of in an environmentally acceptable manner, but frequently they are below the flammability limit and hence will not sustain a fire. One frequently employed solution to this problem is the use of catalytic incineration wherein the mixture of air and toxic organic matter is passed through an oxidation catalyst.
In an article by H. J. Richter et al., Reversibility of Combustion Processes, Efficiency and Costing, Second Law Analysis of Processes, ACS Symposium Series 235, pp. 71-85 (1983), an alternative to fire is proposed in combustion processes. The teachings of this article are restricted to providing improvement of thermodynamic efficiency, however, with no teaching or suggestion of any means for improving heat transfer.
Fluid bed combustion is also an alternative to fire. In some applications, fluid bed combustion can provide better heat transfer than can fire. In other applications, there are substantial heat transfer problems that the use of fluid bed combustion does not avoid.
One example of an application with substantial heat transfer problems is the industrial process known as steam reforming in which hydrogen is produced by passing steam and a hydrocarbon through a nickel catalyst. Typically this is done at temperatures in the range of about 700.degree. C. to 800.degree. C. and at pressures of about 100 to 700 psig. These conditions are too severe for the use of reaction vessels made of mild steel or even stainless steel. Despite their great cost, inconel or some other high nickel alloy must be used. Furthermore, heat must be supplied since the reaction is highly endothermic. While the heat needed can readily be generated by burning fuel, transferring this heat to where it is needed is a problem since the catalyst is in the form of a packed bed. Packed beds are poor conductors of heat and the outer sections of the bed tend to insulate the inner sections. In order to get an adequate rate of heat transfer to the interior of the reaction vessel, the reaction vessels used are long narrow tubes. Thus, to get an adequate rate of heat transfer it is necessary to use very large amounts of expensive alloy tubing.
To avoid this disadvantage there have been proposals to do what is called "adiabatic" steam reforming. Department of Defense Report Number AD-A134224, Evaluation of Adiabatic Reformer in Mixed-Gas-Cycle, by the Power Systems Division of United Technologies Corporation (1983), is a typical example of this technology. In this approach, the heat necessary for the endothermic steam reforming reaction is provided by adding some air to the steam hydrocarbon mixture passing through the reactor. The oxygen in the air reacts with the hydrocarbon, liberating heat. Unfortunately, however, combustible mixtures either ignite or they do not. If ignition does not occur, the needed heat is not liberated. If ignition does occur, the heat is not liberated throughout the reactor where it is needed but at the point of ignition. Since the heat is not liberated uniformly throughout the reactor, there is again a severe heat transfer problem.
The gasification of coal with water is, like steam reforming, an endothermic reaction. A proposal for the improvement of this endothermic reaction has been advanced in an article by G. P. Curran et al., CO.sub.2 Acceptor Gasification Process, Fuel Gasification Symposium, ACS Advances in Chemistry Series 69, Chapter 10, pp. 141-165 (1966). In this article, which is typical of the art, the use of CaO as an acceptor for CO.sub.2 is suggested. The reaction CO.sub.2 +CaO=CaCO.sub.3 is highly exothermic thereby supplying the heat consumed by the endothermic gasification reaction. Furthermore, CO.sub.2 and CO are in equilibrium via the water gas shift reaction H.sub.2 O+CO=CO.sub.2 +H.sub.2. Consequently, removing the CO.sub.2 has the effect of also removing the CO, allowing the production of a gas containing a large mole fraction of hydrogen. Unfortunately, however, for this process to be practical it is necessary to reconvert the CaCO.sub.3 back to CaO. While the heat necessary to do this could readily be generated by burning some fuel, transferring that heat to where it is needed is again a difficult and expensive problem.
Heat transfer is also a substantial problem in other industrial processes in which packed bed reactors are used to carry out endothermic reactions. Examples of such reactions include but are not limited to the cracking of ammonia to make hydrogen/nitrogen mixtures, the gasification of biomass, the catalytic reforming of petroleum hydrocarbons, and the decomposition of methanol.
Another group of applications in which heat transfer is a substantial problem involves the use of packed beds of sorbents. Typically, a gas containing some impurity is passed through the packed bed, the impurity being removed by a sorbent through adsorption or absorption. When the sorbent approaches saturation with the impurity, the sorbent must be regenerated. This is commonly done by heating the packed bed to drive out the impurity. Since, however, the outer portions of the bed tend to insulate the inner portions, heat transfer is not effectively achieved.
Yet another example of a technological problem for which presently available combustion and heat transfer technology do not provide a satisfactory solution is the production of shale oil. The United States has vast reserves of what is commonly referred to as oil shale, i.e., deposits of rock which yield oil when sufficiently heated. No economically acceptable method of producing oil from this resource is presently available because of the limitations of presently available combustion and heat transfer technology. For example, when shale rock is placed into a packed bed retort and heat is supplied to the exterior of the retort, the outer layers of the shale rock insulate the inner layers. This results in unacceptably slow rates of heat transfer and liberation of the oil from the rock. In principal, use of a fluid bed retort would provide a much higher rate of heat transfer, but once shale rock is retorted, it has a tendency to crumble into fine powder. This fine powder tends to fly out of the fluid bed, making operation of the process quite difficult.
From the examples above, it is clear that there is a need in the art for a new method of burning fuel which allows more effective heat transfer than is possible with fire and the presently available alternatives to fire, and accomplishes this without increasing emissions of pollutants.
In the work of R. K. Lyon described in U.S. Pat. Nos. 5,339,754 and 5,509,362 (hereafter the "Lyon Patents"), a method is described for improving heat transfer by using a method called unmixed combustion. In unmixed combustion, a metal is dispersed on a high surface area support. When this metal is exposed to air or a gas containing oxygen, the metal is oxidized producing a significant amount of heat. The gaseous product of this reaction is air that has been depleted of the oxygen consumed by reaction with the metal. Subsequently a gaseous organic fuel is passed over the hot metal oxide. Reaction between the fuel and the metal oxide results in the oxidation of the fuel to produce CO.sub.2 and H.sub.2 O, with the simultaneous production of additional heat and chemical reduction of the metal back to the original state.
In the process described in the Lyon Patents, a single metal is used, and although it is suggested that mixtures of metals may be used, no example of this is provided, nor is any benefit of using mixtures of metals described. Examples of metals described by the Lyon Patents include silver, copper, iron, and nickel. Depending on the process temperature, the fuel oxidation step may either be exothermic, thermoneutral, or endothermic. In general, the metal oxidation step will be strongly exothermic. Furthermore, in the process described in the Lyon Patents, the metal undergoes a transition between the metal in its standard state and one or more oxidation states of the metal. For many applications, this would not be a deficiency. However, in several applications, this would be a deficiency by prohibiting practical use of the process, reducing process efficiency, or requiring that the process be complicated by the addition of heat exchange and heat transfer surfaces which the process of the Lyon Patents is in part intended to avoid.
For example, in the case of the production of hydrogen from diesel fuel in a single step, copending U.S. application Ser. No. 08/428,032 to Lyon, issued as U.S. Pat. No. 5,827,496 on Oct. 27, 1998, the disclosure of which is incorporated by reference, teaches that this process can be conducted by using nickel and nickel oxide, respectively, as the metal and metal oxide. During the first step of the Lyon process, nickel oxide and a fuel, such as methane or a petroleum distillate, react to produce some of the energy required to permit the reforming reaction between additional fuel and steam, fuel+H.sub.2 O=CO+H.sub.2 and the subsequent water-gas shift reaction, CO+H.sub.2 O=CO.sub.2 +H.sub.2, to produce hydrogen. The remaining energy required for the production of hydrogen from fuel and steam is provided by the reaction between CO.sub.2 and CaO to form CaCO.sub.3. This total process can in fact be shown to produce more energy than is required to cause the efficient production of hydrogen. The excess energy is carried away as sensible heat of the product gases. During a subsequent regeneration step, air is allowed to react with the nickel to produce nickel oxide. The energy produced by this reaction is sufficient to cause the CaCO.sub.3 to decompose.
However, it can be shown that while the energy produced in the above Lyon process is sufficient for the decomposition of CaCO.sub.3, the resulting temperature of the reactor will not be high enough to permit the decomposition of all of the CaCO.sub.3. In other words, the conditions in the reactor as described in the Lyon Patents do not thermodynamically favor the complete decomposition of the CaCO.sub.3. Therefore, unless an additional source of heat is supplied to the reactor during the regeneration step, for example, by external heating by the effluent gases from a second reactor that is simultaneously undergoing the reforming step, all of the CaCO.sub.3 will not be decomposed. During subsequent reforming and regeneration steps, the CaO will be slowly consumed and will be unable to react with CO.sub.2, thereby shutting the reaction down. There exists a method for avoiding this deficiency within the Lyon process as described, which is to reduce that portion of the total fuel that is to be converted to hydrogen. Thus, the amount of CaCO.sub.3 produced will be lower relative to the amount of nickel oxide that is reduced to nickel metal, and the heat released during the oxidation of the nickel metal will be sufficient to decompose the smaller amount of CaCO.sub.3. However, this will reduce the thermodynamic efficiency of the total process.